1. Field of the Invention
This invention relates to the field of process rolls, more particularly it pertains to heat transfer rolls for use in the production and processing of sheets of material, such as paper, plastic and rubber.
2. Description of the Prior Art
The principal techniques for manufacturing wide sheets of polymer, such as plastic, or of paper are an extrusion process and a cast film process. The flat sheet extrusion process is used to produce plastic sheet by pressing molten polymer material between two or more rolls that serve to flatten the material into a continuous sheet having a desired thickness. The material passes around and between multiple rolls during production and processing of the sheet. Often a pull roll is used to keep tension in the extruded flat sheet as it exits the final roll. The sheet is then continuously rolled on a core, or it is cut and stacked in flat sheets.
U.S. Pat. No. 5,567,448 describes a heat transfer roll for use in forming flat sheet material by an extrusion process. The roll includes a core, a shell surrounding the core and a duct, through which fluid for controlling the temperature of the sheet flows from the core to the shell. The roll extrudes sheet having a uniform thickness across its width and provides one cooling fluid flow passageway. The roll has no provision for multiple fluid channels and cannot accommodate variable sheet thickness or a variable heat content of sheet material across the width.
Various techniques have been employed to control the temperature on the surface of a roll used to form sheets of paper or plastic. For example, U.S. Pat. No. 4,233,011 describes a roll having a cylindrical core, a tubular shell surrounding the core, and a heating strip carried on the core. Temperature sensors located on the core are used to produce a signal employed by an electrical regulating circuit that controls the application of electric power supplied to the heating strip. The heating strip is regulated so that a predetermined temperature difference between the heated and unheated sides of the core is maintained. The purpose of maintaining this temperature difference on the core is to produce thermal displacement or arching of the core. That displacement is transmitted to the shell. U.S. Pat. No. 5,103,542 describes a fluid distribution system for a variable-crown roll that includes a stationary central axis and a revolving shell surrounding the axis. The fluid distribution system includes a system of ducts in which pressurized fluid enters isolated areas on the roll. The fluid distribution system includes axial ducts and transverse bores that direct fluid to hydraulic loading elements to compensate for stresses resulting during processing. Neither of these patent references describes the use of multiple temperature channels on a forming roll.
Rolls for forming sheet material have conventionally included an annular passage located between a core and a shell surrounding the core. Fluid for cooling the sheet material flows in the annular passage along a spiral path bounded by partition strips that extend radially between the core and the inner surface of the outer shell. U.S. Pat. Nos. 3,548,929 and 3,676,910 describe rolls having a spiral fluid flow channel. The '910 patent describes a machine for forming T-shaped fins that include a sealing gasket, the fins being used as a spiral seal between the core and outer shell of a fluid heat exchanger type roll. The '929 patent describes use of a continuous partition strip arranged in a spiral and located in an annular space between the core and outer shell of the roll. The partition strip is formed of a composite structure that can withstand the chemical and thermal action of certain fluids used for heat transfer purposes that tend to corrode or decompose partitions made of rubber and plastic.
The process for producing long, wide, thin sheet material of plastic, paper and similar materials by the cast film processing includes use of an extruder that delivers molten material in a fluid state to a die. The die has a profiled opening or orifice that forms the surface contour of the sheet as the molten polymer passes through the die orifice to form an elongated sheet width. The sheet may have relatively thick areas spaced across the elongated width and extending continuously along the length of the sheet.
Accordingly, the rate of cooling of the cast sheet product varies across the sheet, that rate being longer in the areas of thick sheet and shorter in the areas of thin sheet. There is need for a heat transfer roll that accommodates the cooling requirement differences across the sheet width by providing multiple cooling channels located within the roll and located appropriately to correspond to the location of thick and thin sheet areas.